feat(runtime): modularize host capabilities

- add an exact-version Capability Registry and structured descriptors
- move Clock bindings, host state, and Linker installation into its own module
- derive required capabilities from Component imports and link only requested interfaces
- cover exact-version resolution and minimal per-component capability sets

crates/wasmeld-runtime/tests/runtime_components.rs

@@ -16,6 +16,7 @@ fn echo_component_round_trips_bytes() {
     let runtime = Runtime::new(RuntimeConfig::default()).expect("runtime should start");
     let (key, actor) = start_component(&runtime, "echo", "echo_component.wasm");
 
+    assert!(runtime.capabilities(&key).unwrap().is_empty());
     assert_eq!(actor.invoke(b"hello wasm".to_vec()).unwrap(), b"hello wasm");
 
     runtime.stop(&key).unwrap();
@@ -171,6 +172,13 @@ fn explicit_clock_capability_is_linked() {
     let runtime = Runtime::new(RuntimeConfig::default()).expect("runtime should start");
     let (key, actor) = start_component(&runtime, "clock-probe", "clock_probe_component.wasm");
 
+    let capabilities = runtime.capabilities(&key).unwrap();
+    assert_eq!(capabilities.len(), 1);
+    assert_eq!(
+        capabilities[0].interface(),
+        "wasmeld:clock/monotonic-clock@0.1.0"
+    );
+
     let first = actor.invoke(b"first".to_vec()).unwrap();
     let second = actor.invoke(b"second".to_vec()).unwrap();
     assert_eq!(&first[8..], b"first");